There are a million places you can go to for information and very often you can get different opinions and “facts” about the same claim. Either evolution is true or it isn’t; the world is either round or it isn’t; vaccines either cause autism or they do not*.
[*The answers are: true, round & no, in case you were wondering.]
Credibility of your sources is something that isn’t always immediately obvious, but in general few things can beat comprehensive research, peer-reviewed and evidence-based theories and finally, consensus in the wider scientific community. This is not to say that theories are never over-turned by brilliant new insights. However, one can make mistakes in judging whether the source of an opinion or story or new theory is credible or not. Here are a few examples to watch out for.
Argument from authority
Trust me, trust me. I’m a doctor.
This gets used remarkably often to make a claim sound more plausible or believable. But even if someone really is an authority in a certain field, it does not make them an authority in all fields. Having a Ph.D. in Literature does not in any way boost your authority or believability about UFOs; being a respected theologian makes you an expert on Scripture, not morality. It does not mean that you can’t express your opinion on these matters, and indeed your opinions might be excellent and lucid. But your argument does not get given extra gravitas simply because you have some expert knowledge on other unrelated subjects.
This type of argument also appears in the ‘Speaking as a’ format:
Speaking as a mother, I think vaccines are dangerous for children.
Sometimes this is used merely for clarification purposes (useful on an anonymous internet forum e.g. I’m a mother so I know what its like to go through six hours of labour); but beware of it being used to give the speaker an added claim to authority that they are not entitled to.
Heresy does not equal correctness
Louis Armstrong sang
They all laughed at Christopher Columbus
When he said the world was round.
Leaving aside the fallacious nature of that line (it was widely accepted that the world was round in Columbus’ day), the message of this song is that sometimes genuine truth goes unaccepted for a long time. However, that does not mean that just because an idea is laughed at, the Discoverer is a martyr to his True Cause. Sometimes the Discoverer is just plain wrong; and that is why people are laughing (or angry). Examples of this are Flat Earth believers, and most conspiracy theorists. The facts need to be collected and checked and proved or disproved before the ‘theory’ can be regarded as valid.
The argument from heresy may be fallacious, but it’s a damn good song.
Argument from mass consensus
Thousands of people believe in ghosts. Therefore ghosts must exist.
There are so many stories about dragons. Therefore there must have been dragons once upon a time.
A thousand years ago you might have said:
Everyone knows the earth is flat
Therefore if you sail your boat too far out, you will go over the edge.
And you still would have been wrong, in both sentences.
Thousands, even millions of people can be wrong.